ncat(1) — Linux manual page


NCAT(1)                     Ncat Reference Guide                     NCAT(1)

NAME         top

       ncat - Concatenate and redirect sockets

SYNOPSIS         top

       ncat [OPTIONS...] [hostname] [port]

DESCRIPTION         top

       Ncat is a feature-packed networking utility which reads and writes
       data across networks from the command line. Ncat was written for the
       Nmap Project and is the culmination of the currently splintered
       family of Netcat incarnations. It is designed to be a reliable
       back-end tool to instantly provide network connectivity to other
       applications and users. Ncat will not only work with IPv4 and IPv6
       but provides the user with a virtually limitless number of potential

       Among Ncat's vast number of features there is the ability to chain
       Ncats together; redirection of TCP, UDP, and SCTP ports to other
       sites; SSL support; and proxy connections via SOCKS4, SOCKS5 or HTTP
       proxies (with optional proxy authentication as well). Some general
       principles apply to most applications and thus give you the
       capability of instantly adding networking support to software that
       would normally never support it.


           Ncat 7.80 ( )
           Usage: ncat [options] [hostname] [port]

           Options taking a time assume seconds. Append 'ms' for milliseconds,
           's' for seconds, 'm' for minutes, or 'h' for hours (e.g. 500ms).
             -4                         Use IPv4 only
             -6                         Use IPv6 only
             -U, --unixsock             Use Unix domain sockets only
                 --vsock                Use vsock sockets only
             -C, --crlf                 Use CRLF for EOL sequence
             -c, --sh-exec <command>    Executes the given command via /bin/sh
             -e, --exec <command>       Executes the given command
                 --lua-exec <filename>  Executes the given Lua script
             -g hop1[,hop2,...]         Loose source routing hop points (8 max)
             -G <n>                     Loose source routing hop pointer (4, 8, 12, ...)
             -m, --max-conns <n>        Maximum <n> simultaneous connections
             -h, --help                 Display this help screen
             -d, --delay <time>         Wait between read/writes
             -o, --output <filename>    Dump session data to a file
             -x, --hex-dump <filename>  Dump session data as hex to a file
             -i, --idle-timeout <time>  Idle read/write timeout
             -p, --source-port port     Specify source port to use
             -s, --source addr          Specify source address to use (doesn't affect -l)
             -l, --listen               Bind and listen for incoming connections
             -k, --keep-open            Accept multiple connections in listen mode
             -n, --nodns                Do not resolve hostnames via DNS
             -t, --telnet               Answer Telnet negotiations
             -u, --udp                  Use UDP instead of default TCP
                 --sctp                 Use SCTP instead of default TCP
             -v, --verbose              Set verbosity level (can be used several times)
             -w, --wait <time>          Connect timeout
             -z                         Zero-I/O mode, report connection status only
                 --append-output        Append rather than clobber specified output files
                 --send-only            Only send data, ignoring received; quit on EOF
                 --recv-only            Only receive data, never send anything
                 --no-shutdown          Continue half-duplex when receiving EOF on stdin
                 --allow                Allow only given hosts to connect to Ncat
                 --allowfile            A file of hosts allowed to connect to Ncat
                 --deny                 Deny given hosts from connecting to Ncat
                 --denyfile             A file of hosts denied from connecting to Ncat
                 --broker               Enable Ncat's connection brokering mode
                 --chat                 Start a simple Ncat chat server
                 --proxy <addr[:port]>  Specify address of host to proxy through
                 --proxy-type <type>    Specify proxy type ("http", "socks4", "socks5")
                 --proxy-auth <auth>    Authenticate with HTTP or SOCKS proxy server
                 --proxy-dns <type>     Specify where to resolve proxy destination
                 --ssl                  Connect or listen with SSL
                 --ssl-cert             Specify SSL certificate file (PEM) for listening
                 --ssl-key              Specify SSL private key (PEM) for listening
                 --ssl-verify           Verify trust and domain name of certificates
                 --ssl-trustfile        PEM file containing trusted SSL certificates
                 --ssl-ciphers          Cipherlist containing SSL ciphers to use
                 --ssl-alpn             ALPN protocol list to use.
                 --version              Display Ncat's version information and exit

           See the ncat(1) manpage for full options, descriptions and usage examples


       Ncat operates in one of two primary modes: connect mode and listen
       mode. Other modes, such as the HTTP proxy server, act as special
       cases of these two. In connect mode, Ncat works as a client. In
       listen mode it is a server.

       In connect mode, the hostname and port arguments tell what to connect
       to.  hostname is required, and may be a hostname or IP address. If
       port is supplied, it must be a decimal port number. If omitted, it
       defaults to 31337.

       In listen mode, hostname and port control the address the server will
       bind to. Both arguments are optional in listen mode. If hostname is
       omitted, it defaults to listening on all available addresses over
       IPv4 and IPv6. If port is omitted, it defaults to 31337.


       -4 (IPv4 only)
           Force the use of IPv4 only.

       -6 (IPv6 only)
           Force the use of IPv6 only.

       -U, --unixsock (Use Unix domain sockets)
           Use Unix domain sockets rather than network sockets. This option
           may be used on its own for stream sockets, or combined with --udp
           for datagram sockets. A description of -U mode is in the section
           called “UNIX DOMAIN SOCKETS”.

       -u, --udp (Use UDP)
           Use UDP for the connection (the default is TCP).

       --sctp (Use SCTP)
           Use SCTP for the connection (the default is TCP). SCTP support is
           implemented in TCP-compatible mode.

       --vsock (Use AF_VSOCK sockets)
           Use AF_VSOCK sockets rather than the default TCP sockets (Linux
           only). This option may be used on its own for stream sockets or
           combined with --udp for datagram sockets. A description of
           --vsock mode is in the section called “AF_VSOCK SOCKETS”.


       -g hop1[,hop2,...] (Loose source routing)
           Sets hops for IPv4 loose source routing. You can use -g once with
           a comma-separated list of hops, use -g multiple times with single
           hops to build the list, or combine the two. Hops can be given as
           IP addresses or hostnames.

       -G ptr (Set source routing pointer)
           Sets the IPv4 source route “pointer” for use with -g. The
           argument must be a multiple of 4 and no more than 28. Not all
           operating systems support setting this pointer to anything other
           than four.

       -p port, --source-port port (Specify source port)
           Set the port number for Ncat to bind to.

       -s host, --source host (Specify source address)
           Set the address for Ncat to bind to.


       See the section called “ACCESS CONTROL OPTIONS” for information on
       limiting the hosts that may connect to the listening Ncat process.

       -l, --listen (Listen for connections)
           Listen for connections rather than connecting to a remote machine

       -m numconns, --max-conns numconns (Specify maximum number of
           The maximum number of simultaneous connections accepted by an
           Ncat instance. 100 is the default (60 on Windows).

       -k, --keep-open (Accept multiple connections)
           Normally a listening server accepts only one connection and then
           quits when the connection is closed. This option makes it accept
           multiple simultaneous connections and wait for more connections
           after they have all been closed. It must be combined with
           --listen. In this mode there is no way for Ncat to know when its
           network input is finished, so it will keep running until
           interrupted. This also means that it will never close its output
           stream, so any program reading from Ncat and looking for
           end-of-file will also hang.

       --broker (Connection brokering)
           Allow multiple parties to connect to a centralised Ncat server
           and communicate with each other. Ncat can broker communication
           between systems that are behind a NAT or otherwise unable to
           directly connect. This option is used in conjunction with
           --listen, which causes the --listen port to have broker mode

       --chat (Ad-hoc “chat server”)
           The --chat option enables chat mode, intended for the exchange of
           text between several users. In chat mode, connection brokering is
           turned on. Ncat prefixes each message received with an ID before
           relaying it to the other connections. The ID is unique for each
           connected client. This helps distinguish who sent what.
           Additionally, non-printing characters such as control characters
           are escaped to keep them from doing damage to a terminal.

SSL OPTIONS         top

       --ssl (Use SSL)
           In connect mode, this option transparently negotiates an SSL
           session with an SSL server to securely encrypt the connection.
           This is particularly handy for talking to SSL enabled HTTP
           servers, etc.

           In server mode, this option listens for incoming SSL connections,
           rather than plain untunneled traffic.

           In UDP connect mode, this option enables Datagram TLS (DTLS).
           This is not supported in server mode.

       --ssl-verify (Verify server certificates)
           In client mode, --ssl-verify is like --ssl except that it also
           requires verification of the server certificate. Ncat comes with
           a default set of trusted certificates in the file ca-bundle.crt.
           Some operating systems provide a default list of trusted
           certificates; these will also be used if available. Use
           --ssl-trustfile to give a custom list. Use -v one or more times
           to get details about verification failures.  Ncat does not check
           for revoked certificates.

           This option has no effect in server mode.

       --ssl-cert certfile.pem (Specify SSL certificate)
           This option gives the location of a PEM-encoded certificate files
           used to authenticate the server (in listen mode) or the client
           (in connect mode). Use it in combination with --ssl-key.

       --ssl-key keyfile.pem (Specify SSL private key)
           This option gives the location of the PEM-encoded private key
           file that goes with the certificate named with --ssl-cert.

       --ssl-trustfile cert.pem (List trusted certificates)
           This option sets a list of certificates that are trusted for
           purposes of certificate verification. It has no effect unless
           combined with --ssl-verify. The argument to this option is the
           name of a PEM file containing trusted certificates. Typically,
           the file will contain certificates of certification authorities,
           though it may also contain server certificates directly. When
           this option is used, Ncat does not use its default certificates.

       --ssl-ciphers cipherlist (Specify SSL ciphersuites)
           This option sets the list of ciphersuites that Ncat will use when
           connecting to servers or when accepting SSL connections from
           clients. The syntax is described in the OpenSSL ciphers(1) man
           page, and defaults to

       --ssl-alpn ALPN list (Specify ALPN protocol list)
           This option allows you to specify a comma-separated list of
           protocols to send via the Application-Layer Protocol Negotiation
           (ALPN) TLS extension. Not supported by all versions of OpenSSL.

PROXY OPTIONS         top

       --proxy host[:port] (Specify proxy address)
           Requests proxying through host:port, using the protocol specified
           by --proxy-type.

           If no port is specified, the proxy protocol's well-known port is
           used (1080 for SOCKS and 3128 for HTTP). When specifying an IPv6
           HTTP proxy server using the IP address rather than the hostname,
           the square-bracket notation (for example [2001:db8::1]:8080) MUST
           be used to separate the port from the IPv6 address. If the proxy
           requires authentication, use --proxy-auth.

       --proxy-type proto (Specify proxy protocol)
           In connect mode, this option requests the protocol proto to
           connect through the proxy host specified by --proxy. In listen
           mode, this option has Ncat act as a proxy server using the
           specified protocol.

           The currently available protocols in connect mode are http
           (CONNECT), socks4 (SOCKSv4), and socks5 (SOCKSv5). The only
           server currently supported is http. If this option is not used,
           the default protocol is http.

       --proxy-auth user[:pass] (Specify proxy credentials)
           In connect mode, gives the credentials that will be used to
           connect to the proxy server. In listen mode, gives the
           credentials that will be required of connecting clients. For use
           with --proxy-type http or --proxy-type socks5, the form should be
           username:password. For --proxy-type socks4, it should be a
           username only.

       --proxy-dns type (Specify where to resolve proxy destination)
           In connect mode, it provides control over whether proxy
           destination hostnames are resolved by the remote proxy server or
           locally, by Ncat itself. Possible values for type are:

           local - Hostnames are resolved locally on the Ncat host. Ncat
           exits with error if the hostname cannot be resolved.

           remote - Hostnames are passed directly onto the remote proxy
           server. This is the default behavior.

           both - Hostname resolution is first attempted on the Ncat host.
           Unresolvable hostnames are passed onto the remote proxy server.

           none - Hostname resolution is completely disabled. Only a literal
           IPv4 or IPv6 address can be used as the proxy destination.

           Local hostname resolution generally respects IP version specified
           with options -4 or -6, except for SOCKS4, which is incompatible
           with IPv6.


       -e command, --exec command (Execute command)
           Execute the specified command after a connection has been
           established. The command must be specified as a full pathname.
           All input from the remote client will be sent to the application
           and responses sent back to the remote client over the socket,
           thus making your command-line application interactive over a
           socket. Combined with --keep-open, Ncat will handle multiple
           simultaneous connections to your specified port/application like
           inetd. Ncat will only accept a maximum, definable, number of
           simultaneous connections controlled by the -m option. By default
           this is set to 100 (60 on Windows).

       -c command, --sh-exec command (Execute command via sh)
           Same as -e, except it tries to execute the command via /bin/sh.
           This means you don't have to specify the full path for the
           command, and shell facilities like environment variables are

       --lua-exec file (Execute a .lua script)
           Runs the specified file as a Lua script after a connection has
           been established, using a built-in interpreter. Both the script's
           standard input and the standard output are redirected to the
           connection data streams.

       All exec options add the following variables to the child's

           The IP address and port number of the remote host. In connect
           mode, it's the target's address; in listen mode, it's the
           client's address.

           The IP address and port number of the local end of the

           The protocol in use: one of TCP, UDP, and SCTP.


       --allow host[,host,...] (Allow connections)
           The list of hosts specified will be the only hosts allowed to
           connect to the Ncat process. All other connection attempts will
           be disconnected. In case of a conflict between --allow and
           --deny, --allow takes precedence. Host specifications follow the
           same syntax used by Nmap.

       --allowfile file (Allow connections from file)
           This has the same functionality as --allow, except that the
           allowed hosts are provided in a new-line delimited allow file,
           rather than directly on the command line.

       --deny host[,host,...] (Deny connections)
           Issue Ncat with a list of hosts that will not be allowed to
           connect to the listening Ncat process. Specified hosts will have
           their session silently terminated if they try to connect. In case
           of a conflict between --allow and --deny, --allow takes
           precedence. Host specifications follow the same syntax used by

       --denyfile file (Deny connections from file)
           This is the same functionality as --deny, except that excluded
           hosts are provided in a new-line delimited deny file, rather than
           directly on the command line.

TIMING OPTIONS         top

       These options accept a time parameter. This is specified in seconds
       by default, though you can append ms, s, m, or h to the value to
       specify milliseconds, seconds, minutes, or hours.

       -d time, --delay time (Specify line delay)
           Set the delay interval for lines sent. This effectively limits
           the number of lines that Ncat will send in the specified period.
           This may be useful for low-bandwidth sites, or have other uses
           such as coping with annoying iptables --limit options.

       -i time, --idle-timeout time (Specify idle timeout)
           Set a fixed timeout for idle connections. If the idle timeout is
           reached, the connection is terminated.

       -w time, --wait time (Specify connect timeout)
           Set a fixed timeout for connection attempts.

OUTPUT OPTIONS         top

       -o file, --output file (Save session data)
           Dump session data to a file

       -x file, --hex-dump file (Save session data in hex)
           Dump session data in hex to a file.

       --append-output (Append output)
           Issue Ncat with --append-ouput along with -o and/or -x and it
           will append the resulted output rather than truncating the
           specified output files.

       -v, --verbose (Be verbose)
           Issue Ncat with -v and it will be verbose and display all kinds
           of useful connection based information. Use more than once (-vv,
           -vvv...) for greater verbosity.

MISC OPTIONS         top

       -C, --crlf (Use CRLF as EOL)
           This option tells Ncat to convert LF line endings to CRLF when
           taking input from standard input.  This is useful for talking to
           some stringent servers directly from a terminal in one of the
           many common plain-text protocols that use CRLF for end-of-line.

       -h, --help (Help screen)
           Displays a short help screen with common options and parameters,
           and then exits.

       --recv-only (Only receive data)
           If this option is passed, Ncat will only receive data and will
           not try to send anything.

       --send-only (Only send data)
           If this option is passed, then Ncat will only send data and will
           ignore anything received. This option also causes Ncat to close
           the network connection and terminate after EOF is received on
           standard input.

       --no-shutdown (Do not shutdown into half-duplex mode)
           If this option is passed, Ncat will not invoke shutdown on a
           socket after seeing EOF on stdin. This is provided for
           backward-compatibility with OpenBSD netcat, which exhibits this
           behavior when executed with its '-d' option.

       -n, --nodns (Do not resolve hostnames)
           Completely disable hostname resolution across all Ncat options,
           such as the destination, source address, source routing hops, and
           the proxy. All addresses must be specified numerically. (Note
           that resolution of proxy destinations is controlled separately
           via option --proxy-dns.)

       -t, --telnet (Answer Telnet negotiations)
           Handle DO/DONT WILL/WONT Telnet negotiations. This makes it
           possible to script Telnet sessions with Ncat.

       --version (Display version)
           Displays the Ncat version number and exits.


       The -U option (same as --unixsock) causes Ncat to use Unix domain
       sockets rather than network sockets. Unix domain sockets exist as an
       entry in the filesystem. You must give the name of a socket to
       connect to or to listen on. For example, to make a connection,

       ncat -U ~/unixsock

       To listen on a socket:

       ncat -l -U ~/unixsock

       Listen mode will create the socket if it doesn't exist. The socket
       will continue to exist after the program ends.

       Both stream and datagram domain sockets are supported. Use -U on its
       own for stream sockets, or combine it with --udp for datagram
       sockets. Datagram sockets require a source socket to connect from. By
       default, a source socket with a random filename will be created as
       needed, and deleted when the program ends. Use the --source with a
       path to use a source socket with a specific name.

AF_VSOCK SOCKETS         top

       The --vsock option causes Ncat to use AF_VSOCK sockets rather than
       network sockets. A CID must be given instead of a hostname or IP
       address. For example, to make a connection to the host,

       ncat --vsock 2 1234

       To listen on a socket:

       ncat -l --vsock 1234

       Both stream and datagram domain sockets are supported, but socket
       type availability depends on the hypervisor. Use --vsock on its own
       for stream sockets, or combine it with --udp for datagram sockets.

EXAMPLES         top

       Connect to on TCP port 8080.
           ncat 8080

       Listen for connections on TCP port 8080.
           ncat -l 8080

       Redirect TCP port 8080 on the local machine to host on port 80.
           ncat --sh-exec "ncat 80" -l 8080 --keep-open

       Bind to TCP port 8081 and attach /bin/bash for the world to access
           ncat --exec "/bin/bash" -l 8081 --keep-open

       Bind a shell to TCP port 8081, limit access to hosts on a local
       network, and limit the maximum number of simultaneous connections to
           ncat --exec "/bin/bash" --max-conns 3 --allow -l
           8081 --keep-open

       Connect to smtphost:25 through a SOCKS4 server on port 1080.
           ncat --proxy socks4host --proxy-type socks4 --proxy-auth joe
           smtphost 25

       Connect to smtphost:25 through a SOCKS5 server on port 1080.
           ncat --proxy socks5host --proxy-type socks5 --proxy-auth
           joe:secret smtphost 25

       Create an HTTP proxy server on localhost port 8888.
           ncat -l --proxy-type http localhost 8888

       Send a file over TCP port 9899 from host2 (client) to host1 (server).
           HOST1$ ncat -l 9899 > outputfile

           HOST2$ ncat HOST1 9899 < inputfile

       Transfer in the other direction, turning Ncat into a “one file”
           HOST1$ ncat -l 9899 < inputfile

           HOST2$ ncat HOST1 9899 > outputfile

EXIT CODE         top

       The exit code reflects whether a connection was made and completed
       successfully. 0 means there was no error. 1 means there was a network
       error of some kind, for example “Connection refused” or “Connection
       reset”. 2 is reserved for all other errors, like an invalid option or
       a nonexistent file.

BUGS         top

       Like its authors, Ncat isn't perfect. But you can help make it better
       by sending bug reports or even writing patches. If Ncat doesn't
       behave the way you expect, first upgrade to the latest version
       available from . If the problem persists, do some
       research to determine whether it has already been discovered and
       addressed. Try Googling the error message or browsing the nmap-dev
       archives at .

       Read this full manual page as well. If nothing comes of this, mail a
       bug report to <>. Please include everything you have
       learned about the problem, as well as what version of Ncat you are
       running and what operating system version it is running on. Problem
       reports and Ncat usage questions sent to are far more
       likely to be answered than those sent to Fyodor directly.

       Code patches to fix bugs are even better than bug reports. Basic
       instructions for creating patch files with your changes are available
       at . Patches may be sent to nmap-dev
       (recommended) or to Fyodor directly.

AUTHORS         top

       ·   Chris Gibson <>

       ·   Kris Katterjohn <>

       ·   Mixter <>

       ·   Fyodor <> ( )

       The original Netcat was written by *Hobbit* <>. While
       Ncat isn't built on any code from the “traditional” Netcat (or any
       other implementation), Ncat is most definitely based on Netcat in
       spirit and functionality.

LEGAL NOTICES         top

   Ncat Copyright and Licensing
       Ncat is (C) 2005–2018 Insecure.Com LLC. It is distributed as free and
       open source software under the same license terms as our Nmap
       software. Precise terms and further details are available from .

   Creative Commons License for this Ncat Guide
       This Ncat Reference Guide is (C) 2005–2018 Insecure.Com LLC. It is
       hereby placed under version 3.0 of the Creative Commons Attribution
       License[1]. This allows you redistribute and modify the work as you
       desire, as long as you credit the original source. Alternatively, you
       may choose to treat this document as falling under the same license
       as Ncap itself (discussed previously).

   Source Code Availability and Community Contributions
       Source is provided to this software because we believe users have a
       right to know exactly what a program is going to do before they run
       it. This also allows you to audit the software for security holes
       (none have been found so far).

       Source code also allows you to port Nmap (which includes Ncat) to new
       platforms, fix bugs, and add new features. You are highly encouraged
       to send your changes to <> for possible incorporation
       into the main distribution. By sending these changes to Fyodor or one
       of the Insecure.Org development mailing lists, it is assumed that you
       are offering the Nmap Project (Insecure.Com LLC) the unlimited,
       non-exclusive right to reuse, modify, and relicense the code. Nmap
       will always be available open source, but this is important because
       the inability to relicense code has caused devastating problems for
       other Free Software projects (such as KDE and NASM). We also
       occasionally relicense the code to third parties as discussed in the
       Nmap man page. If you wish to specify special license conditions of
       your contributions, just say so when you send them.

   No Warranty
       This program is distributed in the hope that it will be useful, but
       WITHOUT ANY WARRANTY; without even the implied warranty of
       General Public License v2.0 for more details at , or in the COPYING file
       included with Nmap.

   Inappropriate Usage
       Ncat should never be installed with special privileges (e.g. suid
       root).  That would open up a major security vulnerability as other
       users on the system (or attackers) could use it for privilege

   Third-Party Software
       This product includes software developed by the Apache Software
       Foundation[2]. A modified version of the Libpcap portable packet
       capture library[3] is distributed along with Ncat. The Windows
       version of Ncat utilized the Libpcap-derived Npcap library[4]
       instead. Certain raw networking functions use the Libdnet[5]
       networking library, which was written by Dug Song.  A modified
       version is distributed with Ncat. Ncat can optionally link with the
       OpenSSL cryptography toolkit[6] for SSL version detection support.
       All of the third-party software described in this paragraph is freely
       redistributable under BSD-style software licenses.

NOTES         top

        1. Creative Commons Attribution License

        2. Apache Software Foundation

        3. Libpcap portable packet capture library

        4. Npcap library

        5. Libdnet

        6. OpenSSL cryptography toolkit

COLOPHON         top

       This page is part of the nmap (a network scanner) project.
       Information about the project can be found at ⟨⟩.  If
       you have a bug report for this manual page, send it to
       This page was obtained from the project's upstream Git mirror of the
       Subversion repository ⟨⟩ on 2020-06-09.
       (At that time, the date of the most recent commit that was found in
       the repository was 2020-06-04.)  If you discover any rendering prob‐
       lems in this HTML version of the page, or you believe there is a bet‐
       ter or more up-to-date source for the page, or you have corrections
       or improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to

Ncat                             08/12/2019                          NCAT(1)